1. Field of the Invention
This invention relates to a motor servo system, and more particularly, a motor servo system which is adapted to carry out a two-position or three-position control of an element to be controlled such as a mechanical element or the like by remote control.
2. Description of the Prior Art
Various kinds of mechanical elements of which a set point to which it is to be moved is predetermined have been conventionally known in the art. For example, a valve mechanism, as shown in FIG. 4, is angularly rotated between 0.degree. and 90.degree. to carry out the passage and shut-off of fluid. Also, in a model airplane controlled by means of a radio control device for remotely controlling a controlled model, a retractable landing gear structure is partially employed which is constructed in a manner to be received in an airframe during the flight as in a real airplane. Such a retractable landing gear is in the category of a mechanical element of which a set point to which it is to be moved is predetermined.
The remote control of such a mechanical element as described above is generally carried out using a servo mechanism. For example, a motor servo circuit for such a retractable landing gear of a model airplane as described above which is remotely controlled by means of a radio control device is constructed in a manner as shown in FIG. 5.
In FIG. 5, reference numeral 51 designates an input terminal to which an input pulse for driving a motor 52 is supplied. More particularly, a pulse which is received and demodulated by a receiver (not shown) and has a pulse width determined depending upon a control input on a transmitter side is supplied in the form of an input signal to the input terminal 51. The input pulse is supplied to a discriminator 53 having an internal reference pulse for discriminating the pulse width. In the discriminator, the input pulse is subjected to judgement as to whether it is a signal for rotating the motor in the right or clockwise direction or in the left or counterclockwise direction based on its pulse width, and is then fed to either an output terminal 53a or an output terminal 53b depending upon the judgement.
Assuming that a difference pulse corresponding to the difference between the input pulse and the internal reference pulse is generated at the output terminal 53a of the discriminator 53 to rotate the motor 52 in the clockwise direction, the difference pulse is subjected to a treatment for suitably extending the pulse width and supplied to a transistor 54a. The difference pulse of which the pulse width has been extended is then fed through an amplifier 55 to the motor 52 to rotate the motor so that a retractable landing gear (not shown) mechanically connected to a revolving shaft of the motor 52 may be received in or drawn out from the body of an airplane.
Also, the revolving shaft of the motor 52 is connected to a movable element 56a of a rotary switch 56. Accordingly, when the movable element 56a is rotated to be contacted with a fixed contact 56b, the base of the transistor 54a is earthed. This results in the transistor 54a being turned off to stop the rotation of the motor 52 to accomplish the movement of the retractable landing gear to a set point. The rotation of the motor 52 in the counterclockwise direction starts when the difference pulse is generated at the output terminal 53b of the discriminator 53, and the retractable landing gear is operated through procedures similar to those described above. In FIG. 5, Vcc designates a driving power source and R designates bias resistances.
The motor servo circuit for operating the mechanical element toward a set point as described above generally employs a system for driving a motor by means of on-off signal of a switch, because it is highly difficult to accomplish the precise coincidence between the operating angle of the mechanical element and the rotating angle of the motor 52. More particularly, the mechanical element stops when it is moved to a set point, and this causes the disagreement between the operating angle of the mechanical element and the rotating angle of the motor, resulting in the supply of an electric current to the motor continuing to incur the waste of power in normal continuous control. Furthermore, the vibration of the mechanical element occurs near the set point due to the overshoot by inertia of the mechanical element or the like.
Thus, the above-described conventional control of a set point of a controlled element such as two-position or three-position control employs on-off control for separating a control loop at the set point or in the vicinity thereof.
However, in order to accomplish the precise control by means of such a conventional system as shown in FIG. 5, it is required to precisely correspond the operating angle of the mechanical element at which the rotary switch 56 is turned on to the width of an input pulse supplied to the input terminal 51. Thus, the conventional motor servo system has a disadvantage that it is highly troublesome to design and construct the rotary switch.
Also, the conventional motor servo system encounters another disadvantage in that the design of the rotary switch 56 must be changed for every variation of the set point of the mechanical element, and any backlash occurring in a power transmission mechanism or the like on the mechanical element side often renders the control according to the width of an input pulse impossible.
Further, in order to overcome the above-described overshoot that is due to inertia of the mechanical element, the conventional system, as shown in FIG. 4, is adapted to interrupt the feed to the motor 52 at the position at which an angle .theta. is left so that the mechanical element may be moved to the set point by inertia. Novertheless, such construction of the conventional system still renders the precise presetting of stop position of the motor 52 highly difficult, because the overshoot by inertia of the mechanical element is varied due to a variation of load as well.
In order to eliminate such difficulties as described above, attempts have been made to adjust the mechanical connecting condition between the revolving shaft of the motor and the mechanical element in various ways. However, this is highly troublesome sufficient to render the precise adjustment substantially impossible.